The 3rd generation partnership project (3GPP) that is a standard organization regarding a mobile communication system is studying new communication systems referred to as long term evolution (LTE) regarding radio sections and system architecture evolution (SAE) regarding the overall system configuration including a core network and a radio access network (hereinafter, collectively referred to as a network as well). This communication system is also referred to as 3.9 generation (3.9 G) system.
As an access scheme of the LTE, orthogonal frequency division multiplexing (OFDM) is used in a downlink direction, and single carrier frequency division multiple access (SC-FDMA) is used in an uplink direction. Further, differently from wideband code division multiple access (W-CDMA) circuit switching is not provided but a packet communication system is only provided in the LTE.
The current decisions by 3GPP regarding the frame configuration in the LTE system described in Non-Patent Document 1 (Chapter 5) will be described with reference to FIG. 1. FIG. 1 is a diagram illustrating the configuration of a radio frame used in the LTE communication system. With reference to FIG. 1, one radio frame is 10 ms. The radio frame is divided into ten equally sized subframes. The subframe is divided into two equally sized slots. The first and sixth subframes contain a downlink synchronization signal (SS) per each radio frame. The synchronization signals are classified into a primary synchronization signal (P-SS) and a secondary synchronization signal (S-SS).
Non-Patent Document 1 (Chapter 5) describes the current decisions by 3GPP regarding the channel configuration in the LTE system. It is assumed that the same channel configuration is used in a closed subscriber group (CSG) cell as that of a non-CSG cell. The CSG cell will be described below (see Chapter 3.1 of Non-Patent Document 2).
A physical broadcast channel (PBCH) is a channel for downlink transmission from a base station to a mobile terminal. A BCH transport block is mapped to four subframes within a 40 ms interval. There is no explicit signaling indicating 40 ms timing.
A physical control format indicator channel (PCFICH) is a channel for downlink transmission from the base station to the mobile terminal. The PCFICH notifies the number of OFDM (Orthogonal Frequency Division Multiplexing) symbols used for PDCCHs from the base station to the mobile terminal. The PCFICH is transmitted in each subframe.
A physical downlink control channel (PDCCH) is a channel for downlink transmission from the base station to the mobile terminal. The PDCCH notifies the resource allocation information for a downlink shared channel (DL-SCH) being one of the transport channels described later, resource allocation information for a paging channel (PCH) being one of the transport channels described later, and HARQ (Hybrid Automatic Repeat reQuest) information related to DL-SCH. The PDCCH carries an uplink scheduling grant. The PDCCH carries acknowledgement (Ack)/negative acknowledgement (Nack) that is a response signal to uplink transmission. The PDCCH is referred to as an L1/L2 control signal as well.
A physical downlink shared channel (PDSCH) is a channel for downlink transmission from the base station to the mobile terminal. A downlink shared channel (DL-SCH) that is a transport channel and a PCH that is a transport channel are mapped to the PDSCH.
A physical multicast channel (PMCH) is a channel for downlink transmission from the base station to the mobile terminal. A multicast channel (MCH) that is a transport channel is mapped to the PMCH.
A physical uplink control channel (PUCCH) is a channel for uplink transmission from the mobile terminal to the base station. The PUCCH carries Ack/Nack that is a response signal to downlink transmission. The PUCCH carries a channel quality indicator (CQI) report. The CQI is quality information indicating the quality of received data or channel quality. In addition, the PUCCH carries a scheduling request (SR).
A physical uplink shared channel (PUSCH) is a channel for uplink transmission from the mobile terminal to the base station. An uplink shared channel (UL-SCH) that is one of the transport channels is mapped to the PUSCH.
A physical hybrid ARQ indicator channel (PHICH) is a channel for downlink transmission from the base station to the mobile terminal. The PHICH carries Ack/Nack that is a response signal to uplink transmission. A physical random access channel (PRACH) is a channel for uplink transmission from the mobile terminal to the base station. The PRACH carries a random access preamble.
A downlink reference signal (RS) is a known symbol in the LTE communication system. The following five types of downlink reference signals are defined: cell-specific reference signals (CRS), MBSFN reference signals, data demodulation reference signal (DM-RS) being UE-specific reference signals, positioning reference signals (PRS), and channel-state information reference signals (CSI-RS). The physical layer measurement objects of a mobile terminal include reference signal received power (RSRP).
The transport channels described in Non-Patent Document 1 (Chapter 5) will be described. A broadcast channel (BCH) among the downlink transport channels shown in FIG. 5(A) is broadcast to the entire coverage of a base station (cell). The BCH is mapped to the physical broadcast channel (PBCH).
Retransmission control according to a hybrid ARQ (HARQ) is applied to a downlink shared channel (DL-SCH). The DL-SCH can be broadcasted to the entire coverage of the base station (cell). The DL-SCH supports dynamic or semi-static resource allocation. The semi-static resource allocation is also referred to as persistent scheduling. The DL-SCH supports discontinuous reception (DRX) of a mobile terminal for enabling the mobile terminal to save power. The DL-SCH is mapped to the physical downlink shared channel (PDSCH).
The paging channel (PCH) supports DRX of the mobile terminal for enabling the mobile terminal to save power. The PCH is required to be broadcast to the entire coverage of the base station (cell). The PCH is mapped to physical resources such as the physical downlink shared channel (PDSCH) that can be used dynamically for traffic.
The multicast channel (MCH) is used for broadcast to the entire coverage of the base station (cell). The MCH supports SFN combining of MBMS services (MTCH and MCCH) in multi-cell transmission. The MCH supports semi-static resource allocation. The MCH is mapped to the PMCH.
Retransmission control according to a hybrid ARQ (HARQ) is applied to an uplink shared channel (UL-SCH) among the uplink transport channels. The UL-SCH supports dynamic or semi-static resource allocation. The UL-SCH is mapped to the physical uplink shared channel (PUSCH).
A random access channel (RACH) is limited to control information. The RACH involves a collision risk. The RACH is mapped to the physical random access channel (PRACH).
The HARQ will be described. The HARQ is the technique for improving the communication quality of a channel by combination of automatic repeat request (ARQ) and error correction (forward error correction). The HARQ is advantageous in that error correction functions effectively by retransmission even for a channel whose communication quality changes. In particular, it is also possible to achieve further quality improvement in retransmission through combination of the reception results of the first transmission and the reception results of the retransmission.
An example of the retransmission method will be described. If the receiver fails to successfully decode the received data, in other words, if a cyclic redundancy check (CRC) error occurs (CRC=NG), the receiver transmits “Nack” to the transmitter. The transmitter that has received “Nack” retransmits the data. If the receiver successfully decodes the received data, in other words, if a CRC error does not occur (CRC=OK), the receiver transmits “AcK” to the transmitter. The transmitter that has received “Ack” transmits the next data.
The logical channels described in Non-Patent Document 1 (Chapter 6) will be described. A broadcast control channel (BCCH) is a downlink channel for broadcast system control information. The BCCH that is a logical channel is mapped to the broadcast channel (BCH) or downlink shared channel (DL-SCH) that is a transport channel.
A paging control channel (PCCH) is a downlink channel for transmitting paging information and system information change notifications. The PCCH is used when the network does not know the cell location of a mobile terminal. The PCCH that is a logical channel is mapped to the paging channel (PCH) that is a transport channel.
A common control channel (CCCH) is a channel for transmission control information between mobile terminals and a base station. The CCCH is used in the case where the mobile terminals have no RRC connection with the network. In a downlink direction, the CCCH is mapped to the downlink shared channel (DL-SCH) that is a transport channel. In an uplink direction, the CCCH is mapped to the uplink shared channel (UL-SCH) that is a transport channel.
A multicast control channel (MCCH) is a downlink channel for point-to-multipoint transmission. The MCCH is used for transmission of MBMS control information for one or several MTCHs from a network to a mobile terminal. The MCCH is used only by a mobile terminal during reception of the MBMS. The MCCH is mapped to the multicast channel (MCH) that is a transport channel.
A dedicated control channel (DCCH) is a channel that transmits dedicated control information between a mobile terminal and a network on a point-to-point basis. The DCCH is used if the mobile terminal has an RRC connection. The DCCH is mapped to the uplink shared channel (UL-SCH) in uplink and mapped to the downlink shared channel (DL-SCH) in downlink.
A dedicated traffic channel (DTCH) is a point-to-point communication channel for transmission of user information to a dedicated mobile terminal. The DTCH exists in uplink as well as downlink. The DTCH is mapped to the uplink shared channel (UL-SCH) in uplink and mapped to the downlink shared channel (DL-SCH) in downlink.
A multicast traffic channel (MTCH) is a downlink channel for traffic data transmission from a network to a mobile terminal. The MTCH is a channel used only by a mobile terminal during reception of the MBMS. The MTCH is mapped to the multicast channel (MCH).
A CGI represents a cell global identifier. An ECGI represents an E-UTRAN cell global identifier. A closed subscriber group (CSG) cell is introduced in the LTE, and the long term evolution advanced (LTE-A) and universal mobile telecommunication system (UMTS) described below.
The closed subscriber group (CSG) cell is a cell in which subscribers who are allowed to use are specified by an operator (hereinafter, also referred to as a “cell for specific subscribers”). The specified subscribers are allowed to access one or more cells of a public land mobile network (PLMN). One or more cells in which the specified subscribers are allowed access are referred to as “CSG cell(s).” Note that access is limited in the PLMN.
The CSG cell is part of the PLMN that broadcasts a specific CSG identity (CSG ID; CSG-ID) and broadcasts “TRUE” in a CSG indication. The authorized members of the subscriber group who have registered in advance access the CSG cells using the CSG-ID that is the access permission information.
The CSG-ID is broadcast by the CSG cell or cells. A plurality of CSG-IDs exist in the LTE communication system. The CSG-IDs are used by mobile terminals (UEs) for making access from CSG-related members easier.
The locations of mobile terminals are tracked on the basis of an area composed of one or more cells. The locations are tracked for enabling tracking the locations of mobile terminals and calling mobile terminals, in other words, incoming calling to mobile terminals even in an idle state. An area for tracking locations of mobile terminals is referred to as a tracking area.
3GPP is studying base stations referred to as Home-NodeB (Home-NB; HNB) and Home-eNodeB (Home-eNB; HeNB). HNB/HeNB is a base station for, for example, household, corporation, or commercial access service in UTRAN/E-UTRAN. Non-Patent Document 3 discloses three different modes of the access to the HeNB and HNB. Specifically, an open access mode, a closed access mode, and a hybrid access mode are disclosed.
The respective modes have the following characteristics. In the open access mode, the HeNB and HNB are operated as a normal cell of a normal operator. In the closed access mode, the HeNB and HNB are operated as a CSG cell. The CSG cell is a CSG cell where only CSG members are allowed access. In the hybrid access mode, the HeNB and HNB are operated as CSG cells where non-CSG members are allowed access at the same time. In other words, a cell in the hybrid access mode (also referred to as a hybrid cell) is the cell that supports both the open access mode and the closed access mode.
In 3GPP, among all physical cell identifiers (PCI), there is a range of PCIs reserved by the network for use by CSG cells (see Chapter 10.5.1.1 of Non-Patent Document 1). Division of the PCI range is also referred to as PCI split. The information about PCI split (also referred to as PCI split information) is broadcast in the system information from a base station to mobile terminals being served thereby. To be served by a base station means to take the base station as a serving cell.
Non-Patent Document 4 discloses the basic operation of a mobile terminal using PCI split. The mobile terminal that does not have the PCI split information needs to perform cell search using all PCIs, for example, using all 504 codes. On the other hand, the mobile terminal that has the PCI split information is capable of performing cell search using the PCI split information.
Further, 3GPP is pursuing specifications standard of long term evolution advanced (LTE-A) as Release 10 (see Non-Patent Documents 5 and 6). The LTE-A is based on the LTE communication system regarding radio sections and is configured by addition of several new techniques thereto.
Carrier aggregation (CA) is studied in the LTE-A system, in which two or more component carriers (CCs) are aggregated to support wider transmission bandwidths up to 100 MHz.
In the case where a CA is configured, a UE has a single RRC connection with a network (NW). In RRC connection, one serving cell provides NAS mobility information and security input. This cell is referred to as a primary cell (PCell). In downlink, a carrier corresponding to a PCell is a downlink primary component carrier (DL PCC). In uplink, a carrier corresponding to PCell is an uplink primary component carrier (UL PCC).
A secondary cell (SCell) is configured to form a serving cell group with a PCell, in accordance with the UE capability. In downlink, a carrier corresponding to SCell is a downlink secondary component carrier (DL SCC). In uplink, a carrier corresponding to SCell is an uplink secondary component carrier (UL SCC).
A serving cell group of one PCell and one or more SCells is configured for one UE.
Further, new techniques in the LTE-A include the technique of supporting wider bands (wider bandwidth extension) and the coordinated multiple point transmission and reception (CoMP) technique. The CoMP studied for LTE-A in 3GPP is described in Non-Patent Document 7.
Further, 3GPP is pursuing specifications standard of Release 12. Among the specifications, in order to satisfy a tremendous volume of traffic in the future, the use of small eNBs configuring a small cell is studied. Examples of the study include the technique of increasing spectral efficiency by configuring a large number of small cells by installing a large number of small eNBs to increase communication capacity.
A traffic amount of a mobile network is in an increasing tendency, and increasing a communication speed is also progressed. When the application of the LTE and the LTE-A is started on a full scale, the communication speed is further increased, and an increase of the traffic amount is expected.
A conventional mobile communication system has a problem that the probability of occurrence of a delay in the network and loss of data may further increase (hereinafter, also referred to as an “occurrence probability”) as a traffic amount increases. A technique to solve such a problem is disclosed in Patent Document 1, for example.
Patent Document 1 discloses a user device having means for selecting a data transmission cell based on a measurement value of a congestion level in a cell.